1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, in particular, to the sheet conveying apparatus which corrects a skew feeding of a sheet and a position in a width direction of the sheet and the image forming apparatus provided with such a sheet conveying apparatus.
2. Description of the Related Art
Generally, with regard to an image forming apparatus, such as a copying machine, a printer and a facsimile apparatus, when a sheet is conveyed in a skewed state or a positional deviation in a sheet-width direction orthogonal to a sheet conveying direction occurs, an image is formed on the sheet in a deviated state relative to a position in which the image is formed. Therefore, a sheet conveying apparatus used in the image forming apparatus includes a skew-feeding correcting portion for correcting the skew feeding and the positional deviation of the sheet under conveyance.
The skew-feeding correcting portion is arranged on an upstream side of a transfer portion which transfers an image to a sheet. For example, the skew-feeding correcting portion performs the correction according to a side registration reference in which a positional deviation of a sheet under conveyance is corrected with respect to a side edge of the sheet (see U.S. Pat. No. 6,273,418).
The skew-feeding correcting portion which corrects a skew feeding of the sheet by using the side registration reference includes a reference portion for abutting (hereinafter, referred to as a reference portion) arranged on one side of a sheet conveying path and in parallel with the conveying direction of the sheet, and plural pairs of skew conveying rollers arranged on the sheet conveying path. The reference portion has a reference surface in approximately-parallel to the sheet conveying direction. The plural pairs of skew conveying rollers are disposed in approximately-parallel with the reference surface and in the sheet conveying direction.
The pairs of skew conveying rollers skew-convey a sheet under conveyance toward the reference portion to force a side edge of the sheet to abut against the reference surface and convey the sheet along the reference surface, and thereby a skew feeding of the sheet relative to the conveying direction is corrected. In addition, a position of the side edge of the sheet in the direction orthogonal to the sheet conveying direction can be defined according to the reference surface, and hence a positional deviation in the sheet-width direction can be corrected based on the reference surface.
When a skew feeding is corrected based on the side registration reference, if a pressure forcing a sheet to abut against the reference surface (a transportation force by the skew conveying rollers) is excessive while a side edge of the sheet abuts against the reference surface, the sheet buckles and a loop is formed. If the sheet in the shape of the loop is conveyed, the loop bends and thereby a sheet jam may occur.
For solving this problem, a guide groove is formed on the reference portion. The guide groove is formed in the sheet conveying direction and can receive a sheet. A side edge of the sheet is inserted into the guide groove and thereby it is prevented that the loop is formed on the side edge of the sheet.
However, regarding the conventional reference portion as mentioned above, a vertical width of the guide groove (hereinafter, referred to as a guide gap) is a width capable of correcting a thickest sheet which can be inserted into the guide groove. Therefore, the guide gap of the guide groove is too wide for a thin sheet, and hence a generation of a loop is not sufficiently prevented.
For solving this problem, a sheet conveying apparatus provided with a gap adjusting unit was proposed (see Japanese Patent Laid-Open No. 2002-356250). The gap adjusting unit can adjust the guide gap of the guide groove formed on the reference portion to meet a thickness of a sheet.
As illustrated in FIG. 7, the above-mentioned conventional reference portion 31A has a slope portion 15A in order to insert a sheet S into a guide groove 9 after adjusting a guide gap G to meet a thickness of the sheet S. The slope portion 15A is formed on an entire opening of the guide groove 9 and widens toward an outside. A side edge of the sheet S can be easily guided to a reference surface positioned on an inside of the guide groove 9 according to the slope portion 15A formed on the entire opening of the guide groove 9.
As illustrated in FIG. 8, the slope portion 15A includes a pair of slope surfaces 16A, 16A, and hence a predetermined space is formed between the slope surfaces 16A, 16A. Accordingly, for example, if a pressure forcing a sheet to abut against the reference portion 31A is excessive, a loop R is formed within the space of the slope portion 15A.
Since a high-stiffness sheet S of which a basis weight is over 60 (gsm) slips on a pair of skew conveying rollers 32 for skew-conveying the sheet S, a loop of the sheet S is hardly formed. However, since a low-stiffness sheet S (ultra thin paper, and the like) of which a basis weight is under 60 (gsm) may not sufficiently slip on the pair of skew conveying rollers 32, a loop of the sheet S is easily formed. This loop R of the sheet S blocks improvements of corrections to a positional deviation in the direction orthogonal to the sheet conveying direction and a skew of the sheet S relative to the sheet conveying direction.
The present invention provides a sheet conveying apparatus and an image forming apparatus provided with the sheet conveying apparatus, in which the sheet conveying apparatus has a skew-feeding correcting portion capable of appropriately eliminating a loop formed on a sheet regardless of the type of sheet to improve an accuracy of correcting a skew feeding.